1. Field of the Invention
The invention relates generally to systems for improving the positioning accuracy of monochromators and particularly relates to calibration systems which compensate for mechanical imperfections in a monochromator's diffraction grating and grating drive assembly.
2. Description of the Relevant Art
Monochromators and their applications are well known in the prior art. In particular, monochromators are known which employ a diffraction grating to disperse a light beam incident on the grating into a spectrum. A system for creating a spectrum using a diffraction grating is shown in U.S Pat. No. 3,973,849, issued Aug. 10, 1976 to Jackson, et ak, entitled SELF-CALIBRATABLE SPECTRUM ANALYZER. The diffraction grating may be rotated by a mechanical drive assembly to assume a position which enables a desired portion of the spectrum to be analyzed.
The width and pitch of the teeth of the diffraction grating, along with other factors, e.g., imperfections in portions of the mechanical drive itself, are capable of introducing significant error into the instrument. This, error affects attempts to accurately position the grating to locate spectral lines of interest. The errors may be of such magnitude as to require large spectral windows to find the desired spectral lines. As a consequence of having to use these large windows the lines used to calibrate the instrument can easily be misidentified, particularly as the number of lines and their density increases.
Prior systems used for calibrating a monochromator to improve positioning accuracy involve making assumptions as to the source (or sources) and nature of error.
For example, one known system utilizes the fact that a specific component of the grating drive assembly, the worm gear, has a particular period of rotation. According to the method employed by the known system, the phase and amplitude of a correction term associated with the period of rotation of the worm gear would be determined and be used to develope an error correction term based on the known source of error.
The prior art technique referred to hereinabove, for calibrating a monochromator with a mechanical grating drive based on a known source of error, calibrates an instrument to within plus or minus 0.015 nm. This system requires approximately 45 to 50 lines to perform the calibration.
It has been determined that it would be desirable to be able to calibrate a monochromator with a mechanical grating drive to within plus or minus 0.002 nm using approximately twice the number of lines used by known systems and bv using small spectral windows.
It has also been determined that it would be desirable to have the operator or analyst in the loop for determining when an instrument is calibrated to an acceptable level and to allow the user to increase the number of lines to be positioned during the calibration process. This is particularly useful in the event preselected lines are rejected during the calibration process and the operator is desirous of using additional lines to calibrate the instrument to a desired level.
Further still, it has been determined that it is desirable to have a system for calibrating a monochromator which does not require making assumptions regarding the source of error, but which inherenly compensates for the largest sources of error identified through use of the system itself.